Revenge of the Fishbones
by Light-Eco-Sage
Summary: It's Genocide. Over and over. Even regular monsters are beginning to sense something is really wrong. Sans and Undyne are both so sickened with Determination across the reset that they can hardly put up fights against the genocidal human anymore. There is only one hope: the two most powerful monsters in the Underground will do everything they can to save the world.


**The Revenge of Fishbones**

 **Author: LightEcoSage**

 **Rated: Teen for language and violence. Hints of romance (Soriel, Alphyne).**

 **Summary: It's a never-ending cycle. Genocide, genocide, genocide. Over and over. Even regular monsters are beginning to sense something is really wrong. A feeling of dread and depression that refuses to lift. Wounds with no known origin that will not heal. Sans and Undyne are both so sickened with Determination poisoning that they can hardly put up fights against the genocidal human abomination anymore. But, perhaps, there is still one last hope for monster-kind. The two most determined monsters in the Underground will have to do everything they can to overpower the fallen child.**

 **Disclaimer: _Undertale_ is owned by Toby Fox.**

 **LES: This story contains a completely corrupted Frisk who no-longer knows how to be merciful.**

* * *

 **Chapter 1: Sans' Proposal**

* * *

Darkness.

Just… nothing except darkness. It seemed to last for an eternity, before the world was suddenly reborn with a scream of pain and terror like a newly born child.

Sans lurched off his mattress, and almost collapsed to the dirty floor as his legs failed him. For several moments, he could do little except lay on the ground, groaning in pain.

It was getting worse. Much worse.

His ribs burned like fire. Magically, he felt like a drained husk. He pushed himself up until he was kneeling, but almost failed at even that. He stared down at the bones that made up his hands. The bones were literally melting before his eyes.

Even without lungs, Sans began to hyperventilate, curling in on himself and focusing his thoughts. f _uck, fuck, fuck. just stay together. keep it together you useless piece of trash._

After several minutes, Sans lifted his shaking hands in front of his eye sockets. They were trembling, but they at least looked normal again. But it still was not good. He pressed his hand to his chest, then pulled it away again with a loud hiss of pain.

His phalanges were dyed red. Not blood. Pure, liquid determination. Poison for monsters, but the only hope that he had of standing up to… that fucking human.

 _i… don't even remember that kid's name anymore._ Sans realized. If he really focused, he could remember a time when they made it to the surface. Had they made it to the surface, once upon a time? Sometimes it seemed like nothing more than a fanciful dream. He could no longer remember when the kid had last smiled, at least with anything except fiendish delight every time they struck another monster down.

He forced himself to his feet, wobbled briefly as his bones tried to start melting again, and slowly hobbled his way to the door of his bedroom.

Just like the last hundred resets, this time lacked Papyrus shouting at him to wake up, but Sans knew exactly where he would find his brother. He opened the door, and gazed down into the living room. Papyrus was there, sitting on the couch, frowning and rubbing at his neck.

Perhaps Papyrus could not exactly remember the resets, but even those who lacked the ability to see past time were starting to feel the effects. Just like the permanent wound on Sans' chest, all of the monsters who were getting killed every single time were starting to find their wounds surviving across the reset. None of them could remember, but now all of the average monsters were constantly sick from their injuries, while boss monsters were all effected by localized wounds and pain.

But things were even worse for Sans and Undyne. Filling themselves with determination over and over again had started to poison them past the reset. At first, it had hardly effected them, but it had gotten worse and worse. She could still put up a pretty good fight against the human, but her illness had lowered her HP down to the point where she couldn't last nearly as long as she used to against the human before melting completely. And Sans? These days it was often a miracle if he could even stand up straight during his battle against the human. He couldn't dodge like he used to. He probably only had the mental energy to slow down time to dodge one or two times before the human sliced through his inflamed ribs for the several hundredth time.

He would fail. He would die. And then everything would reset so that he could fail and die again.

As Sans slumped down the stairs, Papyrus looked up and gasped in shock. "SANS? SANS?! WHAT'S WRONG? WHAT'S THAT RED STUFF ON YOUR SHIRT?"

"it's… nothing." Sans replied, emotionless.

"SANS… YOU LOOK REALLY SICK. IF YOU NEED TO STAY HOME TODAY, I'M SURE UNDYNE WILL UNDERSTAND." Papyrus said.

A hundred resets ago, Sans would have been surprised at Papyrus' willingness to let him be a lazybones. But this had become the new normal for their morning.

"i… i'm fine." Sans lied, his eyes adverted. Sans had been lying to his brother a long time, but being unable to meet his eyes was a new thing. Every time he looked at his brother these days, he could see nothing except a dusty red scarf abandoned in the snow. Papyrus was one of the only things worth living for in this fucked up world, and the human was even taking that away from him.

And his other friends… the Lady behind the door… Sans struggled to remember her name. o _h, yeah, she's toriel… former queen of the monsters. i… i had fallen in love with her, once upon a time._ Those were distant memories, and the wonderful feelings that thinking about her that he'd once cherished were now nothing more then a gray and distant memory. He hadn't even seen her face in years, and she would be dead in a few short hours, maybe less depending on how quickly the kid could work at killing every single monster in the ruins. He couldn't even bring himself to meet her for their last pun-off anymore. For about the last dozen resets he hadn't even been able to meet her from being so sick.

And Alphys… how many times had he gone to check her and the other monsters that had hidden in the true lab from the human to find that she had killed herself, leaving nothing but a smear of dust on the ground?

Asgore? Was he able to put up any fight against the human after Sans fell? Judging from the way he was suffering from a chest wound just like Sans, he too had been killed multiple times.

Even the weed had become subdued. Sans hardly ever saw the weed these days, but even it seemed to be effected. Sure, it still went through the motions, just like everyone, but even it seemed to grow more and more nihilistic.

"SANS? SANS? WHERE ARE YOU GOING?" Papyrus called in shock as Sans walked out of the door with hardly a word to his brother. Before his brother could follow him, Sans gathered what little will power he had left and teleported away.

* * *

When Sans arrived at his destination, he knew that he didn't have much longer. He collapsed under his own weight and it took all of his willpower to not simply fall apart and turn to melted magical goop at that very moment. For a brief moment, he considered simply letting go and falling down. At least then he wouldn't have to live through seeing his brother and all his friends die again. But something kept him going. A tiny thought that seemed to surface from the depths of his memories.

Determination was a poison for monsters, but it was also the source of the kid's powers. Perhaps a bigger source of determination then the kid's would be able to usurp their control of the timeline? But no monster could hope to hold that much determination alone. Sure, he would miss meeting the kid outside the ruins doors, but he didn't give a fuck about that anymore. Following the script no longer mattered. He was hours away from death, even without the kid slicing open his ribs again.

It was time to try something different.

Sans forced himself to his feet, groaned, and fell against Undyne's door. Thankfully, he didn't have to knock. The sound of his skull hitting her door was enough to draw Undyne's attention. Sans knew it would draw her attention. Just about every other monster was depressed, but her illness just made Undyne jumpy and more alert then ever.

She opened the door just as Sans was able to force himself back to his feet. Undyne stood in her doorway, looking a little worse for wear. Her poisoning was not as bad as Sans' yet, but he could tell from looking at her that she was definitely sick. She was slumped over slightly, and several of her scales had loosened and fallen out since her skin was not as solid as it should be.

But she was all business when she saw Sans on her doorstep, and the state he was in. "Sans! What the fuck? What the hell happened to you?" She demanded.

Of course, it was still too early for word to have reached her. The spiders in the ruins had always been able to get word to Undyne through Muffet if the human killed anyone in the ruins, but it was still too early for that telegram to reach Undyne. The kid was probably just starting their murder spree over again, or would be soon. Sans had never seen what went on behind the ruins doors, so how could he know exactly how long it took before the killing started?

"sup, undyne. i… i need your help." Sans said, almost falling into Undyne's house. Undyne just barely managed to catch him before he fell to the ground again.

"Damn, Sans!" Undyne gasped. "What do you need… uh… what the fuck is that red stuff on your shirt?!" Before Sans could protest, Undyne had ripped his shirt right down the middle, exposing his rib cage to her eyes. Her eyes widened at the vicious-looking wound across his rib cage. "What… what the fuck is this, Sans? Who did this to you? How are you not… dead?" She asked, a little shocked. She knew about Sans' low HP, and any wound like this would have definitely taken much more then a single HP off any monster.

"you're… not going to believe me." Sans began, allowing Undyne to help him over to the table.

But Undyne, despite her hard personality, did care deeply for every single one of the monsters who worked under her, even the sentries. Unbelievable or not, she could not deny the evidence of her own eyes. The wound on Sans' chest was very real, so whatever was bothering him was very real. "What's going on?" She asked.

Sans sighed and began the explanation that he had not shared with anyone else in hundreds of resets.

* * *

Once the explanation was done, Sans was left with nothing but to hope that Undyne believed him about the resets and the determination poison that they were currently suffering from. She had not said a single word through his explanation, but just listened to him silently.

Finally, she sighed. "I believe you." She said.

"you… you do?" Sans asked, a little shocked. The last time he'd tried to tell someone about the resets, it had been treated like a joke.

"Yeah." Undyne said. "Now that you say it, I can feel it. Nightmares… and my body..." She clutched her arms tighter. "I feel like I'm… falling apart? When I get up in the morning, I feel like it takes several minutes where I feel like my body is about to fall apart."

"that's the determination." Sans said, holding up his hand for her to inspect. Undyne could see how his joints were starting to melt and fuse together.

"Is this happening to all of the monsters?" Undyne asked.

"the determination poisoning?" Sans clarified. "nah. that's just you and me. none of the other monsters are able to be tap into any amounts of determination, let alone able to use it to fight. but..." Sans paused, flexing his phalanges and trying to get his joints working again. "even we have begun to reach our limit. i certainly have reached mine. i can't fight the human anymore, not by myself. i'll have melted long before they reach the hall of judgment."

Undyne sighed. "I… you're right, Sans. I know that I'm not feeling my best right now. I'm not sure if I could take on a powerful and determined human right now."

"i know. But, undyne, there is still something we can do. You and I are the strongest, and there is still one thing left to do."

"And what is that?" Undyne asked.

"everything, all of this, has to do with determination. that is what gives the human their power, to control the timeline, to keep coming back even after death. but the power is fickle. if there is a being in the underground with more determination then the human, then that being would take over control of the timeline. that being could perform a reset. and without the ability to reset, we would only have to kill the human one time for it to be permanent. we'd have the seventh human soul. we could leave the underground." Sans explained.

"That sounds like something we should try. But why haven't you come to anyone with this plan before?" Undyne asked.

"for two reasons. the first reason is that for a long time, i could hold the human off in battle. i… i thought i could talk to them… change their mind. but my words no longer reach them and they have memorized my attack patterns. i used to be able to kill them hundreds of times for every time they killed me. but now I can't even touch them." Sans confessed. "the second reason is… because if it doesn't work… if there is not enough determination… then there will be no going back."

"What exactly is your plan?" Undyne asked. "You kind of sound like you want to pour all of the determination possible into a single monster. But how would that be possible if the monster would just end up melting?"

"it's not possible." Sans said. "this is my plan… we are all ready melting, undyne. it's too late for the two of us. but we are the two most determined monsters in the underground. and, failing that, i know of a place that is filled with monsters also filled with determination. if we… meld all of these sources of determination together, we just might be able to overpower the human's determination… and maybe even finally be able to kill them."

"Meld?" Undyne asked. "I'm not sure I like that sound of that."

"it's… exactly what it sounds like." Sans said with a sigh. "when two or more monsters are melting from determination, they can fuse their bodies together. we call them 'amalgamates'. amalgamates are nearly immortal, or at least cannot be attacked physically. and the best part is that although they deteriorate in power over time, when they are newly fused they are as powerful as the sum of their parts."

"And if we can't gather enough determination?" Undyne asked.

"then we will be stuck that way until the human resets and there will be no hope for us to break the time loop." Sans said.

"Okay, but what about if we do get the reset?" Undyne asked. "When the reset happens, we will unfuse and the human will be the one with the most determination again."

"...not if we extract it from them." Sans said. "determination is a strange thing. it's not effected by resetting like everything else. once it is extracted from the human's soul, they will never have it again unless it is returned, even over a reset."

"Okay. How do we extract it?" Undyne asked.

Sans sighed, and held out his hand. "come with me. i don't have the energy for more than a couple of teleports, but we'll need to go to the true lab. and i don't think i could walk there anymore, even if we take the Riverperson's boat."

Undyne had some idea of Sans' ability to teleport. She knew that he managed to hold down several sentry stations spread across the entire Underground. Without the ability to teleport, there was no way he could do all of his jobs. But she had never teleported with him before. As far as she knew, he only had teleported his brother. Never the less, she took his hand. "Very well. If things are as serious as you say, I'll go with you."

"okay. just… give me a moment..." Sans trailed off, it took him a while to gather up the power needed to teleport, but then in a flash they were gone.

* * *

(The True Lab)

As soon as they landed, Sans almost collapsed again, but Undyne managed to catch him. "t—thanks." Sans grumbled, getting back on his feet.

"Where… where are we?" Undyne asked, looking around and staring in shock at the large machine shaped like a skull that was hanging above a dark void.

Sans was not sure how truthful he wanted to be. Undyne is in love with Alphys, and he knew Alphys well enough that she would want to tell Undyne about her involvement with this place herself when she was ready. "this… this is my dad's old lab." Sans said. Well… it wasn't a lie. "he's the one who experimented with determination, and the one who invented this machine… among others." He gestured towards the skull-shaped machine. "this is a machine designed to extract determination straight from a human soul. the plan is this… we fuse ourselves together, supplementing ourselves with the amalgamates in this lab if we need to. gain control of the reset function. and then we teleport to the human's location, kill them, take their soul, bring it back here to extract the determination, and then reset. then everyone will be alive again, we will no longer be fused, and the human will have no determination left to control the timeline. so it should be an easy task to kill the human, take their soul, and use it to help break the barrier."

"Will the human soul still work to break the barrier without their determination?" Undyne asked. "I thought that was the thing that made human souls more powerful than monster souls. Would it be strong enough to count as another human soul?"

"i… i don't know." Sans admitted. "if that's the case we'll just kill them and wait for another human. failing to capture a human soul is a better option then everyone dying."

"Yes. I agree." Undyne said.

"Wow." A voice suddenly spoke out from the darkness. "I never knew that you could be so devious, smiley trashbag. This is new. I'll admit it. This is new."

Undyne jumped to attention, but Sans hardly reacted at all. Of course the weed was here. Even if it couldn't be seen, it was nearly always around watching things. "it's fine, undyne… i think..." He said, he turned one glowing eye on the flower. "so you were listening? if you try to stop us..." Even to Sans, it felt like a hollow threat. He could barely stand, let alone kill even a weed.

"Ha!" Flowey laughed as he appeared from between the tiles at their feet. "That's a pretty good joke, ya comedian. Your best yet, I'd say. You can barely stay balanced on your feet, let alone fight me." The weed glared up at Sans with a vicious grin. "Oh… how I want to kill you right now and wipe that dumb smile off your face once and for all. You've broken the rules, strayed from the script, so there is nothing protecting you from me killing you right now!" The weed's grin grew wider as it formed a ring of bullets around Sans. Even a single touch would kill him and he couldn't spare any more energy for a teleport, especially not so soon after teleporting. After a few tense seconds, the bullets disappeared. "But… as much as I'd hate to admit it… you're right about one thing. The kid needs to be stopped."

"really? i'm kinda shocked." Sans was genuinely surprised. "i always thought you were on the human's side."

To Sans' shock, the flower actually seemed to wilt a little bit. And now that Sans was seeing the flower up close, he could see the unmistakable signs of damage on the flower, just the same as any other monster in the Underground right now. The weed had been killed repeatedly as well. "Allies mean about as much to the kid as enemies." The weed admitted. "And I may be soulless, but there's only so many times I can stand being crushed into dust over and over. Both Asgore and I… we never even get the chance to fight back and defend ourselves. So I'll defend myself now." The entire lab started to rumble and, to Sans' shock, the weed began to pull in several vines, all of them holding the amalgamates. Sans knew many of them, and it hurt to see them so scared and confused. But, maybe they would not be needed?

"you're… helping us?" Sans asked, a little shocked. "are you going to… i know that you have determination as well."

"Do I look like a half-melted freak like you?" The weed sneered. "This flower body _can_ handle my determination, so I'm not melting and can't become an amalgamate. But I'll… _lend_ you some of my determination if this is not enough? After all… I had the most before the kid got here, so with my help this will definitely work."

"wow. I never thought i'd say this but i'm actually considering not killing you." Sans said. "thanks."

"Yeah, yeah. Just get on with it. The kid is about to fight Papyrus."

"The human has all ready reached Papyrus?" Undyne asked, shocked. "But that's impossible! Even if they walked that distance without fighting at all, there's no way they could have made it to Snowdin by now! Let alone hunting down everyone."

"i'm afraid that might be my fault." Sans groaned. "when I shortcut here i _may_ have also transported us through time as well."

"Time?!" Undyne repeated in shock.

"then we need to go. there's no time left." Sans said.

"So… how do we… do this thing?" Undyne asked.

"well… i'm more melted then you are." Sans said, pointing out the obvious. "so i suppose i'll just join my body with yours. you are physically the stronger one anyway."

"Ugh… this is so fuckin' weird." Undyne groaned.

The more determination a monster had, the fast the degeneration. So Sans allowed his mind to travel back to all those times in the Hall of Judgment. To bring back the feeling of knowing that everything depended on him. That all of time would soon be destroyed. To know that he had to stop the time loop no matter the cost to himself. But, instead of forcing himself to stay together through sheer mental will, he allowed his body to start to melt.

"Oh fuck oh fuck oh fuck..." Undyne mumbled to herself as Sans began to melt right before her eyes.

Just before Sans' body lost all physicality, he reached towards Undyne and, after a brief moment of hesitation, Undyne meet Sans' hand.

Flowey stayed back and wordlessly watched as the two most powerful fighters in the Underground merged into a single being. And as they fused, Flowey thought for the first time in years that there was a chance for every single one of them to get out of this fucking time loop. Sure, watching the kid run genocide had been fun at first. He hadn't even minded getting killed over and over. But just like watching a goodie-two-shoes pacifist run over and over gets boring, the genocide got boring too. It was the same thing over and over. The fusion of Undyne and Sans into a single being had _never_ happened before. It was new and interesting, and Flowey was determined to see where it would end.

Of course, two monsters together would definitely not be able to equal the determination of a single human. _Oh well…_ Flowey shrugged. _That's what we have the other amalgamates for..._


End file.
